Background: IgG4-related disease is a systemic lymphoproliferative syndrome that shows IgG4-producing plasma cell expansion in affected organs with fibrotic or sclerotic changes. The lacrimal glands, salivary glands and pancreas are typically affected. We report a case of IgG4-related disease presenting a soft tissue tumor affecting skeletal muscle. Case Report: A 32-year-old man presented a soft tissue mass in his left arm. Magnetic resonance imaging revealed a spindle like, peripheral mass, in the lateral head of the triceps of his left arm. Tru-Cut Biopsy provided the diagnosis of IgG4-related disease affecting skeletal muscle. Glucocorticoid treatment was effective. Conclusion: To our knowledge, this is the first reported case of IgG4-related disease affecting skeletal muscle and presenting a soft tissue mass.
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